For many years now displays have been designed for multiple users and optimised so that viewers can see the same good image quality from different angles with respect to the display. This assumes that the multiple users require the same information from the display. However, there are many applications where it would be desirable for the individual users to be able to see different information from the same display. For example in an automobile, the driver may wish to view satellite navigation data while the passenger may wish to view a movie. If two displays were used in this instance, it would be possible for the driver to view the movie, which might be distracting, and providing two displays would take up extra space and increase cost. In computer games, each player may wish to view the game from his or her own perspective. This is currently done by each player viewing their unique perspective on individual screens. This takes up a lot of space and is not practical for portable games.
By showing more than one image to more than one user on one display, there can be a considerable saving in space and cost. This may be a desirable thing in airplanes where each passenger is provided with their own video screen. By providing one central screen for two or more passengers but retaining the ability to select their own movie, there could be a considerable saving in cost, space and weight. There is also the ability to preclude the users from seeing each other's views. This might be a desirable thing in security applications such as banking or sales transactions as well as games.
In normal vision, the two human eyes perceive views of the world from different perspectives due to their separate location within the head. These two perspectives are then used by the brain to assess the distance to various objects in a scene. In order to build a display which will effectively display a three dimensional image, it is necessary to re-create this situation and supply a so-called “stereoscopic pair” of images, one to each eye of the observer.
Three-dimensional displays are classified into two types depending on the method used to supply the different views to the eyes. Stereoscopic displays typically display both of the images over a wide viewing area. However, each of the views is encoded, for instance by colour, polarisation state or time of display, so that a filter system of glasses worn by the observer can separate the views and will only let each eye see the view that is intended for it.
Autostereoscopic displays require no viewing aids to be worn by the observer but the two views are only visible from defined regions of space. The region of space in which an image is visible across the whole of the display active area is termed a “viewing region”. If the observer is situated such that one of their eyes is in one viewing region and the other eye is in the viewing region for the other image of the pair, then a correct set of views will be seen and a three-dimensional image will be perceived.
For flat panel autostereoscopic displays, the formation of the viewing regions is typically due to a combination of the pixel structure of the display unit and an optical element, generically termed a parallax optic. An example of such an optic is a parallax barrier. This element is a screen with vertical transmissive slits separated by opaque regions. This screen can be set in front of a spatial light modulator (SLM) with a two-dimensional array of pixel apertures as shown in FIG. 1.
The display comprises a transmissive spatial light modulator in the form of a liquid crystal device (LCD) comprising an active matrix thin film transistor (TFT) substrate 1, a counter substrate 2, a pixel (picture element) plane 3 formed by a liquid crystal layer, polarisers 4 and viewing angle enhancement films 5. The SLM is illuminated by a backlight (not shown) with illumination in the direction indicated by an arrow 6. The display is of the front parallax barrier type and comprises a parallax barrier having a substrate 7, an aperture array 8 and an anti-reflection (AR) coating 9.
The SLM is arranged such that columns of pixels are provided extending vertically for normal viewing with the columns having a horizontal pitch p. The parallax barrier provides an array 8 of apertures or slits with the slits being parallel to each other and extending parallel to the pixel columns. The slits have a width 2w and a horizontal pitch b and are spaced from the pixel plane 3 by a separation s.
The display has an intended viewing distance ro with left and right viewing windows 10 and 11 at the widest parts of the viewing regions defining a window plane 12. The viewing windows 10 and 11 have a pitch e which is generally made substantially equal to the typical or average human eye separation. The centre of each primary viewing window 10, 11 subtends a half angle α to the display normal.
The pitch b of the slits in the parallax barrier is chosen to be close to an integer multiple of the pixel pitch p of the SLM so that groups of columns of pixels are associated with a specific slit of the parallax barrier. FIG. 1 shows an SLM in which two pixel columns are associated with each slit of the parallax barrier.
FIG. 2 of the accompanying drawings shows the angular zones of light created from an SLM and parallax barrier where the parallax barrier has a pitch b of an exact integer multiple of the pixel column pitch p. In this case, the angular zones coming from different locations across the display panel surface intermix and a pure zone of view for image 1 or image 2 does not exist. In order to address this, the pitch b of the parallax optic is reduced slightly so that the angular zones converge at the window plane 12 in front of the display. This change in the parallax optic pitch is termed “viewpoint correction” and is shown in FIG. 3 of the accompanying drawings. The viewing regions created in this way are roughly kite shaped.
For a colour display, each pixel is generally provided with a filter associated with one of the three primary colours. By controlling groups of three pixels each with a different colour filter, substantially all visible colours may be produced. In an autostereoscopic display, each of the stereoscopic image “channels” must contain a sufficient number of colour filters for a balanced colour output. Many SLMs have colour filters arranged in vertical columns, due to ease of manufacture, so that all the pixels in a given column have the same colour filter associated with them. If a parallax optic is used with such an SLM such that three pixel columns are associated with each slit (or lenslet), only one colour will be visible in each viewing region. This may be avoided using, for example, the techniques disclosed in EP 0 752 610.
The function of the parallax optic is to restrict the light transmitted through the pixels to certain output angles. This restriction defines the angle of view of each of the pixel columns behind a given slit. The angular range of view of each pixel is decided by the refractive index of the glass n, the pixel width p and the separation between the pixel and the parallax optic planes s, and is given by
      sin    ⁢                  ⁢    α    =      n    ⁢                  ⁢          sin      ⁡              (                  arctan          ⁡                      (                          p                              2                ⁢                s                                      )                          )            
In order to increase the angle between viewing windows, it is necessary to either increase the pixel pitch p, decrease the gap between the parallax optic and the pixels s, or increase the refractive index of the glass n. Changing any of these variables is not easy. It is not always practical or cost effective to significantly change the refractive index of the substrate glass. Pixel pitch is typically defined by the required resolution specification of the panel and therefore cannot be changed. Additionally increasing pixel pitch requires a similar increase in the parallax barrier pitch which makes the barrier more visible, thus detracting from the final image quality. Decreasing s results in manufacturing problems associated with making and handling thin glass. Therefore, it is difficult to use a standard parallax barrier to create 3D or multi-view displays with wide viewing angles.
One option for increasing the pixel pitch and therefore the viewing angle is to rotate the pixel configuration such that the colour subpixels run horizontally rather than vertically as described in JP7-28015. This results in a threefold increase in pixel width and therefore an increase by roughly three times in viewing angle. As mentioned above, this has the disadvantage that the barrier pitch increases as pixel pitch increases which in turn increases the visibility. The manufacture and driving of such a non-standard panel may not be cost effective. Additionally, there may be applications in which the increase in viewing angle needs to be greater than three times the standard configuration and therefore simply rotating the pixels may not be enough.
The window plane defines the optimum viewing distance of the display. An observer whose eyes are located in this plane will receive the best performance of the display. As they move laterally in this plane, the image on the display will remain until they reach the edge of the viewing region, whereupon the whole display will swiftly change to the next image as the eye moves into the adjacent viewing region. The line of the window plane within a viewing region is sometimes termed a “viewing window”.
In an ideal display of the type shown in FIGS. 1, 2 and 3, the intensity distribution of light across each viewing window would be a “top hat” function. In other words, for each viewing window, the light intensity would be constant across the viewing window and zero outside the viewing window in the viewing plane. However, degradation of the window intensity distribution occurs so that the lateral and longitudinal viewing freedom of the observer is reduced compared with that illustrated in FIG. 3. This can be caused by diffraction through the apertures as well as by gaps between pixels resulting in dark regions at the edges of the windows. In an ideal display, right eye image data would not be present in the left eye viewing region and vice versa. However, in practice, crosstalk occurs so that each eye can see some of the light intended for the other eye.
While a particular parallax element (slit or lenslet) is principally associated with one group of pixel columns, the adjacent groups of pixel columns will also be imaged by the element. Imaging of these groups creates lobes of repeated viewing regions to either side of the central, or zero order, lobe. These lobes repeat all the properties of the central lobe but are in general affected to a larger extent by the imperfections and aberrations of the optical system and will eventually become unusable as the lobe order increases. It is possible to use these higher order lobes in order to achieve a wider angle between windows. However, there will be a significant reduction in performance.
For example, by using a parallax optic having a pitch of b˜3p, it is possible to use windows A and C in FIG. 4a of the accompanying drawings, for either 3D viewing or “dual view” where window B is either black or contains arbitrary data. While this gives an increased angle of view, the brightness is reduced to ⅔ of that produced by a barrier having a pitch b˜2p since every third pixel is “viewed” in an unused window. The increase in barrier pitch would increase the visibility of the barrier and the horizontal resolution would decrease. The windows are smaller than those of a two view system with the same viewing angle as illustrated in FIG. 4b of the accompanying drawings, and therefore viewing freedom would be reduced. Another problem with this example is that, if the colour filters are arranged in RGB columns, then each window only sees one colour “subpixel”.
FIG. 5 of the accompanying drawings illustrates a parallax barrier with a slit width=3p and spaced with a pitch b˜6p. This results in windows which are at a separation angle of three times that of a standard parallax barrier of pitch b˜2p. Because the slit width is 3p, all three colour filter colours 15, 16 and 17 are seen in the same proportion. However, this is only true at the exact centre of the viewing window as illustrated in FIG. 5a of the accompanying drawings. As soon as the viewer moves away from the centre of the viewing window, the neighbouring image data 15a will be seen and crosstalk will occur as shown in FIG. 5b of the accompanying drawings.
In order to reduce this crosstalk, the slit width can be reduced to less than 3p. However, this results in an uneven colour balance. As shown in FIG. 6a of the accompanying drawings, when the viewer is in the centre of the viewing window, the “white” pixel will appear light green because more of the green sub-pixel 16 is visible than each of the red and blue sub-pixels 15 and 17. Because of the reduced slit width, it is possible for the viewer to move away from the contre of the viewing window without seeing the neighbouring image data. However, the colour balance changes with angle of view and, as shown in FIG. 6b of the accompanying drawings for movement in one direction, the “white” pixel will now appear cyan.
Another way to achieve a larger separation angle would be to use the standard b˜2p barrier and use both secondary lobes A and D in FIG. 7a of the accompanying drawings, which are separated by two window widths. This would have the same barrier visibility, brightness, and horizontal resolution as the two view system shown in FIG. 7b of the accompanying drawings with the same angle. However, the viewing freedom would be significantly reduced.
Another known type of directional display is the rear parallax barrier display as shown in FIG. 8 of the accompanying drawings. In this case, the parallax barrier 7, 8 is placed behind the SLM 1 to 5 i.e. between the SLM and the backlight. This arrangement has the advantage that the barrier is kept behind the SLM away from possible damage.
Lenticular screens are used to direct interlaced images to multiple directions, which can be designed to give a 3D image or give multiple images in multiple directions. Practical lenses tend to suffer from scatter and poor anti-reflection performance so that the surface is very visible in both ambient and backlit environments. Therefore, the image quality of lenticular screens can be poor and the system suffers from similar problems as parallax barriers such as the need for close proximity to the image pixels.
Holographic methods of image splitting also exist but they suffer from viewing angle problems, pseudoscopic zones and a lack of easy control of the images.
Micropolariser displays use a polarised directional light source and patterned high precision micropolariser elements aligned with the LCD pixels. Such a display offers the potential for high window image quality as well as 2D/3D function in a compact package. The dominant requirement is the incorporation into the LCD of micropolariser elements to avoid parallax issues.
U.S. Pat. No. 6,424,323 discloses an image deflection system comprising a lenticular screen overlying a display device. The display is controlled to provide at least two independent images to be viewed from different viewing positions.
JP 7-28015 discloses the use of a patterned pixel shape with a lenticular barrier in which the windows formed by the display have a minimum crosstalk. Crosstalk is reduced by moving the relative positions of the pixels and appropriately arranging the spacing and orientation.
Other known types of multiple view displays are disclosed in WO 98/27451, DE 19822342 and JP H7-104212.
JP-A-8-36145 discloses a parallax barrier in which the slits have pitches which are randomly chosen from a plurality of predetermined pitches. The chosen pitches may be repeated as groups across the barrier.
GB-2352573 discloses a parallax barrier in which the slits are spaced apart uniformly with each slit comprising a plurality of sub-apertures.